Showing papers on "Coke published in 2012"

The Chemistry and Technology of Coal

Abstract: Part I Character and Properties Occurrence and Resources Introduction History Rock Environment Seam Structure Reserves Resources References Classification Introduction Classification Systems Correlation of the Various Systems Epilogue References An Organic Sediment Introduction Precursors of Coal Coal-Forming Processes Heteroatoms in Coal Epilogue References An Organic Rock Introduction Physical Structure Petrology Petrography Petrology, Petrography, and Behavior References Recovery Introduction Exploratory Drilling Equipment and Techniques Mining References Preparation, Transportation, and Storage Introduction Coal Preparation Size Reduction Coal Cleaning Coal Drying Desulfurization Coal Sampling Transportation Storage References Mineral Matter Introduction Origin of Mineral Matter in Coal Occurrence Mineral Types Classification Evaluation of Mineral Matter Chemistry of Ash Formation Effect of Mineral Matter in Coal References Coal Analysis Introduction Sampling Proximate Analysis Ultimate Analysis (Elemental Analysis) Calorific Value Reporting Coal Analyses Precision and Accuracy Interrelationships of Analytical and Physical Data References Coal Properties Introduction Physical Properties Mechanical Properties Thermal Properties Electrical Properties Epilogue References Organic Constituents Introduction Solvent Extraction Chemical Methods Spectroscopic Methods Physical Property Methods Heteroatoms Molecular Weight Assessment of Coal Structure References Solvent Extraction Introduction Physicochemical Concepts Action of Specific Solvents Composition of the Extracts Solvolysis Solvent Swelling of Coal Epilogue References Chemical Reactivity Introduction Reactions with Oxygen (or Air) Reactions with Oxidants Bacterial Oxidation of Coal Hydrogenation Halogenation Alkylation Depolymerization Hydrolysis Miscellaneous Reactions References Thermal Reactivity Introduction Thermal Decomposition Processes Physicochemical Aspects Thermal Decomposition Products References Part II Technology and Utilization Combustion Introduction Chemistry and Physics Combustion Systems Coal-Liquid Mixtures Coal Combustion Products References Electric Power Generation Introduction Electricity from Coal Steam Generation Power Plant Waste Coal-Water Fuels Air Pollution Control Devices Carbon Capture and Storage Technologies References Carbonization Introduction Physicochemical Aspects Process Concepts Coal Tar Coke Charcoal References Briquetting and Pelletizing Introduction General Concepts Briquetting Technology Charcoal Briquettes Bio-Briquettes Briquette Properties Processes Pelletizing References Liquefaction Introduction Physicochemical Aspects Process Classification Reactors Products References Liquefaction Processes Introduction Pyrolysis Processes Solvent Extraction Processes Catalytic Liquefaction Processes Indirect Liquefaction Processes Coal Liquids Refinery References Gasification Introduction Gaseous Products Physicochemical Aspects Process Types and Reactors Gasification of Coal with Biomass and Waste Underground Gasification Environmental Aspects References Gasification Processes Introduction Fixed-Bed Processes Fluidized-Bed Processes Entrained-Bed Processes Molten Salt Processes Underground Gasification References Clean Coal Technologies Introduction Historical Perspectives Modern Perspectives Clean Coal Technology Managing Wastes from Coal Use Carbon Dioxide Capture and Sequestration References Gas Cleaning Introduction Environmental Legislation General Aspects Particulate Matter Removal Acid Gas Removal Removal of Sulfur-Containing Gases Removal of Nitrogen-Containing Gases References Chemicals from Coal Historical Aspects Coal Tar Chemicals Fischer-Tropsch Chemicals Chemicals from Methane References Environmental Aspects of Coal Use Introduction Production Preparation Transportation and Storage Combustion Carbonization Liquefaction Gasification Clean Coal Technologies References Coal and Energy Security Introduction Energy Security National Energy Plan and Coal Utilization Electric Power Generation Hydrogen from Coal Energy Security and Sustainable Development References Glossary Index

Effect of desilication of H-ZSM-5 by alkali treatment on catalytic performance in hexane cracking

Abstract: The effects of external surface and acid properties of desilicated H-type ZSM-5 zeolites (H-ZSM-5) on their catalytic performance in hexane cracking were investigated. H-ZSM-5 with two different crystallite sizes of 100 nm and 1 μm were treated with NaOH solution of different concentrations. The external surface area (SEXT) was increased with an increase in the NaOH concentration, because of the formation of mesopores inside the H-ZSM-5 crystallites as a result of desilication. The increase in the SEXT of the H-ZSM-5 catalysts contributed to mitigating the catalyst deactivation during the hexane cracking. Although the amount of coke deposited on the alkali-treated H-ZSM-5 was larger than that on the parent H-ZSM-5, the micropore volume of the alkali-treated H-ZSM-5 decreased less due to coke deposition than that of the parent. The deactivation rate and the decrease in the micropore volume of the small-sized H-ZSM-5 catalysts were smaller than those of the large-sized catalysts, because they had shorter average diffusion path lengths. Thus the activity of the alkali-treated H-ZSM-5, especially small-sized one is less sensitive to coke deposition. Lewis acid sites (LASs) were generated by treating with NaOH of high concentrations. The selectivities to benzene, toluene and xylene (BTX) in the hexane cracking were increased with an increase in the LASs amount at high reaction temperatures (≥873 K). The LASs on the alkali-treated H-ZSM-5 were selectively removed by acid treatment. The resultant H-ZSM-5 exhibited a slightly lower hexane conversion and a lower selectivity to BTX but a small amount of coke compared to the parent and alkali-treated H-ZSM-5 catalysts, suggesting that LASs on alkali-treated H-ZSM-5 accelerated the dehydrogenation including hydride transfer and aromatization, forming BTX, which would be precursors of coke.

DFT Study of the Water–Gas Shift Reaction and Coke Formation on Ni(111) and Ni(211) Surfaces

Abstract: We report for the first time results of a systematic density functional theory (DFT) study of the water-gas shift (WGS) reaction and coke formation pathways on Ni(111) and Ni(211) surfaces, consisting of 21 elementary-like steps and 12 surface species. Bronsted–Evans–Polanyi correlations are proposed for dehydrogenation and C–O bond breaking reactions on flat and stepped surfaces. The DFT results suggest that the flat surface is slightly more active for the WGS reaction, which occurs mainly via the carboxyl pathway with the CO* + OH* ⇌ COOH*+* as the rate determining step. On the stepped surface, beyond the carboxyl pathway, the DFT energetics indicates that a parallel route via formate and formyl intermediates is favored. Ni(111) has a much lower activity for C–O bond breaking, and thus, flat surfaces are less susceptible to deactivation by coke.

Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst

Abstract: A series of supported Ni catalysts was prepared using various supports and tested in low-temperature (400 °C) dry reforming of methane. Ni/La 2 O 3 -ZrO 2 showed near-to equilibrium yields of CO and H 2 and the highest stability. This catalyst was studied in greater detail in order to determine the influence of the support morphology on the catalyst activity and stability. To this end, nonstructured, mesoporous, and macroporous La 2 O 3 -ZrO 2 were prepared and characterized before and after loading with Ni. Among them, only Ni on the mesoporous support showed practically no change in activity over 180 h on-stream, whereas the others deactivated. Formation of graphene-like coke layers on the catalysts and of NiO shell over Ni particles appears to be responsible for catalyst deactivation, while sintering of Ni did not play a major role. The enhanced stability of Ni on the mesoporous La 2 O 3 -ZrO 2 is attributed to stronger interaction of the Ni particles with the support stemming from a pore confinement effect.

A highly active and coke-resistant steam reforming catalyst comprising uniform nickel-iron alloy nanoparticles

Abstract: Doing fine with Ni-Fe: The calcination and reduction of a hydrotalcite precursor containing Ni and Fe ions gives uniform Ni-Fe alloy nanoparticles mixed with Mg(Ni, Fe, Al)O particles. The uniformity of the Ni-Fe alloy nanoparticles is connected to the catalyst's high activity and resistance to coke formation in toluene and phenol steam reforming reactions.

Steam reforming of acetic acid over Ni/ZrO2 catalysts: Effects of nickel loading and particle size on product distribution and coke formation

Abstract: Steam reforming of acetic acid has been carried out over a series of Ni/ZrO 2 catalysts to measure the effects of nickel loading on distribution of the reforming products and coke formation. Ni (≤13 wt.%)/ZrO 2 catalysts do not contain enough active metal sites for steam reforming of both acetic acid and organic by-products. Ni (≥20 wt.%)/ZrO 2 catalysts can effectively catalyze steam reforming but lack selectivity, since methanation and reverse water gas shift reactions are promoted, leading to low hydrogen yields. Ni (16 wt.%)/ZrO 2 catalyst is the most selective one, due to its low activity to the secondary reactions that contribute to by-product production. Coke formation is suppressed with the increase of nickel loading up to 16 wt.%, and then restarts to increases with the further increase of nickel loading. Polymerization of acetone is the main route for coke deposition over the Ni (≤13 wt.%)/ZrO 2 catalysts. Methane decomposition and CO disproportion are the two main routes for coke formation over the Ni (≥20 wt.%)/ZrO 2 catalysts, and methane contributes more to coke formation than CO. In addition, activity of Ni/ZrO 2 catalyst towards the secondary reactions such as methanation, reverse water gas shift reaction, methane decomposition, and CO disproportion are closely related to nickel loading and nickel particle sizes.

Kinetics of CO2/Coal Gasification in Molten Blast Furnace Slag

Abstract: The coal/CO2 gasification reactions in molten BF (blast furnace) slag were studied kinetically by temperature-programmed thermogravimetry using a thermal analyzer. The effect of heating rates and molten BF slag on coal gasification were studied, and the activation energies, frequency factors, and most possibility mechanism functions were calculated. The results show that the order of reactivity sequence at these temperatures was DT (Datong) coal > FX (Fuxin) coal > coke. With the increase in heating rate, the carbon conversion, and the peak value of reaction rate increased at the same reaction time, the carbon conversion curve shifts to a higher temperature and the reaction rate curve shifts rightward systematically, both of the time required for the carbon conversion to reach nearly unity and the time necessary for reaction rate to reach its maximum decreased. The carbon conversion and reaction rates were sensitive to BF slag; at the same time, the carbon conversion and reaction rates of coal gasificatio...

Ni/Al coprecipitated catalysts modified with magnesium and copper for the catalytic steam reforming of model compounds from biomass pyrolysis liquids

Abstract: Ni/Al coprecipitated catalysts modified with magnesium and copper have been prepared by a constant pH technique and tested in the catalytic steam reforming of model compounds (acetic acid, acetol and butanol) from biomass pyrolysis liquids at 650 °C and atmospheric pressure. Catalysts with different copper contents, reduced at 650 °C for 1 h, were tested in the steam reforming of acetic acid with a steam/carbon (S/C) molar ratio of 5.6. The best performance and the highest hydrogen yield in these conditions were achieved with the 5% Cu catalyst. This catalyst reduced at 650 °C during 10 h showed a high activity, close to the thermodynamic equilibrium, and a stable performance during 12 h in the steam reforming of acetic acid with a S/C = 5.6, using a short space time of 1.00 g catalyst min/g acetic acid. Copper as a promoter produces counterbalanced effects: a decrease in the initial reforming activity and an enhancement of the catalyst stability. The initial steam reforming activity decreased and the CH 4 yield increased concurrently with increasing the copper content, because of the Ni dilution effect. Copper has a positive effect inhibiting the formation of encapsulating coke, identified as the cause for deactivation in acetic acid steam reforming with a steam-to-carbon molar ratio (S/C) of 5.6. However, such a positive effect of copper has not been observed in acetic acid steam reforming with S/C = 14.7 or in the steam reforming of acetol and butanol.

How to obtain a reliable structural characterization of polished graphitized carbons by Raman microspectroscopy

Abstract: A series of graphitized carbon materials, produced by the pyrolysis of an anthracene-based coke at temperatures ranging from 1600 to 2900 °C, were studied by Raman microspectroscopy to assess the applicability of this technique to the particular case of polished carbon materials. The polishing process was shown to change significantly the first-order Raman spectra (D band intensity increase) and therefore to induce unacceptable errors in the characterization of the intrinsic structure of these materials. The deconvolution of Raman spectra, related to the unpolished graphitized carbons at varying temperatures, highlighted a linear relationship between the intensity ratio ID/IG and the G band width. Thus, as the latter appears to be insensitive to the polishing, we highly recommend using it for a reliable assessment of the intrinsic structural disorder of polished carbon materials. Copyright © 2011 John Wiley & Sons, Ltd.

Recycling of waste plastics into fuels. LDPE conversion in FCC

Abstract: In order to study the tertiary recycling of waste polymers in standard FCC units low density polyethylene (LDPE) was dissolved into a commercial vacuum gas oil at 2 and 6 wt.% and converted over two equilibrium FCC catalysts of the octane-barrel and resid types in a CREC Riser Simulator laboratory reactor. The reaction temperatures were 500, 525 and 550 °C, the mass catalyst to oil relationship was 6.35 and the contact times were from 3 to 30 s. The study included the effect of the concentration of LDPE over conversion, the various product (dry gas, LPG, gasoline, LCO and coke) yields and selectivities. Results were very similar for the two concentrations. At typical conversions of 70 wt.%, dry gas and gasoline yields increased about 10 wt.%, LPG yields between 9 and 13 wt.%, LCO yields decreased more than 15 wt.% and coke yields were lower than 7.7 wt.% The RON index of gasoline was improved slightly (up to one point), mainly due to significant increases in olefin concentrations, while the fuel quality of the LCO cut was not affected. LDPE is easily converted and seems to be subjected to primary reactions of catalytic cracking, thus increasing the yields of olefins in the LPG and gasoline boiling ranges. It was concluded that recycling waste LDPE by co-processing it as part of conventional feeds to the FCC would not interfere with the standard operation.

Microwave-assisted pyrolysis of HDPE using an activated carbon bed

Abstract: Microwave assisted pyrolysis of high density polyethylene (HDPE) using a reactor bed of catalytic activated carbon produces a condensed liquid product with a carbon chain length profile matching petrol and diesel. Greater cracking was observed across all operating temperatures, and a lighter liquid product with a narrower range of chain lengths was produced compared to the use of a bed of traditional coke.

Influence of the shape of Ni catalysts in the glycerol steam reforming

Abstract: Biomass is an alternative to replace the use of fossil fuels. Glycerol, a byproduct in the biodiesel production, can be used for obtaining hydrogen. The most efficient method for obtaining hydrogen from glycerol is the steam reforming (SR). So far all the published papers report the use of conventional catalyst. In this paper, a structured catalyst has been prepared and compared with the conventional ones (powder and spherical pellets). Results show that the structured catalyst (monolith) is more stable as formation of coke was not observed.

Steam catalytic cracking of naphtha over ZSM-5 zeolite for production of propene and ethene: Micro and macroscopic implications of the presence of steam

Abstract: One option to produce more ethene and propene can be to crack naphtha type fractions in dedicated smaller FCC units. We present here the results obtained for high temperature steam catalytic cracking (SCC) of a representative naphtha product (n-heptane) with ZSM-5. It has been found that under those conditions the presence of steam produces an irreversible dealumination of the zeolite as well as a reversible deactivation due to the interaction of water with active sites with a negative effect on protolytic cracking. A kinetic decay model that takes into account the two phenomena has been developed. The apparent activation energy is lower in the presence of steam. It appears that whilst the presence of steam is vital when processing heavy feeds to achieve a better feed dispersion and a more effective catalytic cracking in conventional fluid catalytic cracking (FCC) units, in the case of steam catalytic cracking of naphtha (n-heptane) the presence of steam has a negative effect on the final performance of the catalyst. On the other hand, whilst steam does not modify ethene and propene selectivity, significantly decreases H2 and CH4 formation, as well as formation of potential coke precursors.

Conversion of methanol to hydrocarbons over conventional and mesoporous H-ZSM-5 and H-Ga-MFI: Major differences in deactivation behavior

Abstract: Methanol has been converted to hydrocarbons over conventional and mesoporous H-ZSM-5 and H-Ga-MFI. The gallium based zeotypes are analogous to H-ZSM-5, but the Bronsted acidity is introduced by framework incorporation of gallium rather than aluminum, which leads to lower intrinsic acid strength. In the methanol-to-hydrocarbons (MTH) process, H-ZSM-5 is subjected to coke formation leading to catalyst deactivation. Here we show that when the gallium containing zeotypes are employed in the MTH process, only insignificant amounts of coke are present in the deactivated catalysts, indicating distinct differences in deactivation mechanisms of the two different catalysts. This is investigated further through FT-IR measurements as well as catalytic experiments employing regenerated and steamed catalysts. From this it is concluded that the H-Ga-MFI is subjected to irreversible deactivation by steaming (hydrolysis) of the Ga O bonds in the zeolite structure rather than coke deposition.

Deactivation and Regeneration of H-USY Zeolite during Lignin Catalytic Fast Pyrolysis

Abstract: Catalytic fast pyrolysis of alkaline lignin over H-USY zeolites uniquely yields a high fraction of aromatics but only very small amounts of tar and char. This study focused on the deactivation and regeneration of the H-USY zeolite. N2 physisorption, thermogravimetric analysis (TGA), temperature-programmed desorption of NH3, and 27Al MAS NMR analysis were used to determine coking, pore topology, and the number of acid sites of the deactivated and regenerated H-USY catalyst. Deactivation occurred mainly owing to coke deposition on the catalyst. Both pore blockage and active site poisoning were observed during deactivation. The coke was removed through calcination of the deactivated catalysts in air at high temperature. Calcination restored the high porosity and the acid sites to a large extent. Even better catalytic performance was obtained after the reintroduction of protons through an ion-exchange step. Recovery of catalytic properties was high but incomplete, because of structural changes in the catalyst during the reaction–regeneration process.

Tar and coke formation during hydrothermal processing of glycerol and glucose. Influence of temperature, residence time and feed concentration

Abstract: The hydrothermal gasification is a promising process for the conversion of wet biomass. Coke formation can occur during the heating up, leading to a decrease in the gasification efficiency or even to a blockage of the plant. A new work-up procedure was developed for separating and measuring the amounts of both tars and coke, combined with a compound-specific HPLC analysis of the aqueous product phase. The glycerol experiments were conducted in a temperature range of 300–430 °C, residence times of 5–120 min and feed concentrations up to 30 wt.% at pressures around 30 MPa. Additional experiments were performed with glucose, phenol, hydroquinone, and ethylene glycol. Coke formation was observed at subcritical temperatures and long residence times. The highest rate of coke formation occurred in the temperature range of 350–370 °C. Phenol and hydroquinone can be ruled out as precursors for the formation of coke.